1. Field of the Invention
This invention relates generally to the reduction of interference signals of known frequency characteristics in the receiver portion of a digital transmission system, and more particularly to the use of a notch filter which precedes an interpolator for reducing AC powerline interference signals from a received digital signal.
2. Description of the Prior Art
In a digital transmission system for transmitting information which was originally gathered in analog form, such as a wireless telemetry system using Pulse Code Modulation (PCM) for an analog information signal, occasional samples of the transmitted information can be lost or otherwise received incorrectly due to poor signal strength, radio frequency interference (RFI), or multi-path effects. When samples are lost, the digital receiver is able to detect the loss by using, for example, a checksum or Cyclic Redundancy Check (CRC) type of parity checking technique. The lost samples are then replaced by approximations using any of several known interpolation techniques which use adjacently received correct samples to approximate the lost ones. In this way, transmission errors can be prevented from causing significant distortion in the received signal.
However, problems are frequently encountered if the transmitted signal contains AC line-frequency interference. For example, in instrumentation including sensitive amplifiers, such as an electrocardiograph, AC line-frequency interference is commonly picked up. To reduce line-frequency interference, it is common to include in the signal processing path a notch filter tuned to the line-frequency. Ideally, this filter should be located in the transmitter portion of the system at a point in the signal path prior to the radio frequency (RF) modulator. However, this location is not preferred in order to save space and cost in the transmitter. That is, it is often desirable to have a transmitter which is as small as possible, which consumes relatively little power (and can therefore have a relatively long operating life using batteries) and be of low cost. To include an analog or digital notch filter in the transmitter adversely impacts all of these desired features for the transmitter. Consequently, the desirable and economic alternative is to include the line-frequency notch filter in the receiver. However, as previously noted, an interpolator is normally included in the initial stages of digital signal processing (to compensate for transmission errors), which presents problems since the interpolator must accurately approximate both the original analog information as well as any picked-up AC line-frequency component which was included in the lost digital sample, in order that the notch filter will properly reduce the line-frequency component. Unfortunately, it is expensive to implement an interpolation scheme that will accurately approximate an interference signal of 50 Hz or 60 Hz when the digital sampling frequency is only about 4 times higher (typically 200 to 300 Hz for ECG signals). An interpolation scheme which can accurately approximate both the amplitude and phase of an interference signal which has a frequency which is greater than 20% of the sampling frequency is relatively complex and expensive to implement both in terms of circuit area, components and power consumption. Additionally, since interpolation tends to attenuate or smooth out higher frequencies from a signal, a relatively low cost implementation of an interpolator will leave "dead-spots" in the amplitude envelope of the line-frequency signal component whenever transmission errors occur. These amplitude dead-spots become noise impulses when the line-frequency is removed by the notch filter, which would be located in the signal processing path after the interpolator, because the correct operation of the filter relies on the assumption that the amplitude of the interference will be relatively constant. When a dead-spot is encountered, the notch filter over-compensates for the reduced amplitude dead-spot, thereby generating an impulse noise.
In order to avoid this problem, one may think that simply placing the notch filter in front of the interpolator would be sufficient. However, in this case, since transmission errors will randomly cause some of the received samples to incorrectly have a very large amplitude, this will perturb the output of the notch filter for several sample periods following each incorrect sample, due to the transient response of the filter, e.g., ringing. The interpolator will provide an estimated sample to correct for each incorrect sample, but cannot compensate for the filter perturbation, which will then generally manifest itself in the form of an exponentially decaying sinusoid superimposed on the interpolator output signal.
It is an object of the present invention to provide a low-cost and more effective solution for reducing the presence of unwanted signals having known frequency characteristics from a received digital signal.